PGP is based on public keys. That means you have a public key that you can freely pass around to anybody. Using your public key they can generate encrypted messages that only you can decode using your secret private key. You never give out your private key. Similarly you can generate messages for others using their public key, which only they can decode by using their secret private keys.
You can also create unique signatures using your private key that will authenticate messages you send for others. Only you with your private key can create such a signature, but anybody with your public key can check if it is valid.
PGP is available for DOS, Mac, and several other systems.
The latest version is 2.6. It is available from MIT.
Check out the PGP FAQ.
There is a book about PGP:
PGP: Pretty Good Privacy by Simson Garfinkel 1st Edition November 1994 (est.) 250 pages (est),ISBN: 1-56592-098-8, $17.95 (est)
Here are some locations of MacPGP 2.2 that I know about:
Host plaza.aarnet.edu.au (IP: 139.130.4.6)
Location: /micros/mac/umich/util/encryption
FILE -r--r--r-- 312726 Mar 14 02:45 macpgp2.2.cpt.hqx
Host src.doc.ic.ac.uk (IP: 146.169.2.1)
Location: /computing/systems/mac/umich/util/encryption
FILE -rw-rw-r-- 287112 Mar 14 02:45 macpgp2.2.cpt.hqx.Z
Host wuarchive.wustl.edu (IP: 128.252.135.4)
Location: /mirrors3/archive.umich.edu/mac/util/encryption
FILE -r--r--r-- 312726 Mar 13 20:45 macpgp2.2.cpt.hqx
Host soda.berkeley.edu (IP: 128.32.149.19)
Location: /pub/cypherpunks/pgp
FILE -rw-r--r-- 312726 Mar 17 07:53 macpgp2.2.cpt.hqx
Host ftp.u.washington.edu (IP: 140.142.56.2)
Location: /pub/user-supported/cypherpunks
FILE -rw-r--r-- 312726 Mar 17 14:25 macpgp2.2.cpt.hqx